1. Field of the Invention
This invention relates to sealing the exhaust gas flow from an internal combustion engine.
2. Prior Art
Using electronic engine control to control an internal combustion engine is known. In particular it is known to use exhaust gas oxygen (EGO) sensors to produce a signal for use by an electronic engine control module.
U.S. Pat. No. 4,842,713 discloses an EGO sensor having a sealing ring 36 made of a soft and deformable metal, such as copper or soft steel. It is also known to use talc to seal of the oxygen sensor as it is screwed into or otherwise fastened into the exhaust gas path. The seal permits proper operation of the exhaust gas oxygen sensor by separating two different gaseous environments.
Limiting gas leakage through the seal keeps hydrocarbons from the exhaust gas out of an air reference cavity which is meant to have a gaseous environment representing the ambient. If leakage exceeds 0.03 cc per minute, the air reference cavity can become tainted and the signal produced by the exhaust gas oxygen sensor which indicates the amount of oxygen in the exhaust gas can be affected. The normal signal ranges from +1 volts DC showing a rich air-to-fuel ratio to 0 volts DC which shows a lean air-to-fuel ratio.
At least three factors can increase the amount of hydrocarbons that leak through the seal into the ambient cavity. The first two factors occur when the engine has been at rest for an extended period of time. The exposure to either gasoline fumes or liquid gasoline particles can increase the leakage. Both seep into the talc which can have an undesirably high gasoline permeability and adsorption. When the exhaust system warms up after start up, the hydrocarbons escape into the air reference cavity as gasoline fumes. The third factor that increases the level of hydrocarbons in the ambient cavity is backpressure. This occurs when the throttle is wide open. The exhaust gas cannot escape the system quickly enough and the pressure backs up from the engine forcing hydrocarbons through the seal.
When leakage exceeds the level at which the signal is affected, the oxygen in the air reference cavity of the oxygen sensor can be depleted and a characteristic shift down can occur. This is a problem in which the signal starts from -1 volts DC when lean and works its way up to 0 volts DC when rich. When an electronic engine control module reads the signal, it assumes that the air-to-fuel ratio is too lean and attempts to rectify the situation by increasing injector pulse width to allow the intake of more fuel into the engine. This adversely affects the emissions. It would be desirable to have a seal with improved electrical isolation and a very low leak rate. These are some of the problems this invention overcomes.